This invention relates to a catalyst for removal of arsenic from hydrocarbon oils and fractions and a method of making such catalyst.
Arsenic is naturally present in petroleum feedstocks. Although the concentration of arsenic is low in most petroleum feeds, some crude oils, especially those found on the West Coast of the United States, contain high levels of arsenic. Some shale oils are also known to have high levels of arsenic. In these and other feedstocks, arsenic is usually found in the form of organo-metallic complexes and is present in light petroleum fractions such as naphtha and distillates. This arsenic is deposited on hydrotreating catalysts during operations designed to remove sulfur and nitrogen compounds.
The deposition of only 0.1 wt. % arsenic on a hydrotreating catalyst can cause significant catalyst deactivation. In some services, as little as 0.5 wt. % arsenic deposited on a catalyst can cause the loss of more than 70% of such catalyst activity. Refiners cope with this problem by regularly replacing conventional hydrotreating catalysts that have been poisoned by arsenic. Heretofore, catalysts having specific activity and selectivity for arsenic removal have not been developed. The present invention provides for a catalyst that is effective at selectively removing arsenic from petroleum feedstocks, particularly from naphtha and light distillates, while simultaneously catalyzing the removal of sulfur and nitrogen.
Some researchers have taught hydrotreating catalysts that bear a superficial similarity to the catalyst of the present invention. For example, in U.S. Pat. No. 4,048,155, O""Hara teaches a desulfurization catalyst comprising an inorganic oxide carrier, a Group VIB metal component and a Group VIII metal component that is prepared by extruding at least 10% of the Group VIII metal component with the inorganic oxide carrier material and then finished by impregnating the resulting extrudate with the Group VIB metal and remaining Group VIII metal components.
More recently, U.S. Pat. Nos. 5,389,595 and 5,494,568 (Simpson, et al.) and U.S. Pat. No. 5,686,375 (Iyer, et al.) have taught a hydrotreating catalyst, hydrotreating process and method of making said catalyst wherein the catalyst comprises a porous refractory support containing an underbedded Group VIII metal-containing compound also impregnated with a Group VIB metal.
It has been found that a catalyst comprising a porous support and high concentrations of a Group VIII metal selected from nickel and cobalt and a Group VIB metal selected from molybdenum and tungsten is particularly effective at removing arsenic from petroleum feedstocks. The present invention provides for such a catalyst comprising a porous support, at least 10 wt. % molybdenum or tungsten calculated as the metal and between 1.5 moles and 2.5 moles of nickel or cobalt for each mole of the Group VIB metal. This catalyst has been found to be particularly effective at removing arsenic from naphtha and light distillate cuts while exhibiting hydrodesulfurization and hydrodemetallization similar to conventional catalysts. The catalyst of the present invention exhibits good stability, i.e., the ability to exhibit activity equivalent to or close to its initial activity, even when the amount of arsenic removed (i.e., deposited on the catalyst) is as high as 3 to 4 wt. % of the catalyst when fresh.
The present invention also provides for a process for manufacturing such catalyst comprising:
a) impregnating a porous support with sufficient solution of a nickel or cobalt compound such that the impregnated support comprises at least 8 wt. % Group VIII metal calculated as the metal;
b) drying the Ni- or Co-impregnated support of step (a) at low temperature and then calcining at a temperature of at least 427xc2x0 C.;
c) impregnating the product of step (b) with a solution of a Group VIB compound selected from molybdenum and tungsten compounds and, optionally, phosphorus and/or an additional amount of the Group VIII compound deposited in step (a), followed by drying; and
d) calcining the product of step (c) at a temperature at least 30xc2x0 C. lower than that at which the calcining in step (b) is done.
Finally, the present invention provides for a catalyst made by the process of the present invention and for a process for removing arsenic from a petroleum fraction comprising contacting such petroleum fraction with the catalyst of the present invention in the presence of hydrogen at elevated temperature and pressure.
Catalyst
The catalyst of the present invention comprises a porous refractory support impregnated with a Group VIB metal selected from molybdenum and tungsten and a Group VIII metal selected from nickel and cobalt, wherein the atomic ratio of Group VIII metal to Group VIB metal is at least 1.5 and the amount of Group VIB metal on the finished catalyst is at least 8 wt. %.
Support
A porous support is typically used to carry the catalytically reactive metal(s). Supports are typically alumina, alumina-silica, silica, titania, zirconia, boria, magnesia, zeolites and combinations thereof. Porous carbon-based materials such as activated carbon and/or porous graphite can be utilized as well. The preferred supports in this invention are alumina-based and alumina-silica-based carriers.
The pore size distribution of the support is relatively immaterial to arsenic removal activity. Therefore, the pore size distribution of the support should be optimized to maximize sulfur and nitrogen removal. Arsenic removal depends primarily upon the amount of Group VIII metal present in the catalyst and proximity of such to the Group VIB metal, when present in a dispersed state on the catalyst surface.
Catalytic Metal
The key element of the present invention is the impregnation of the support with high concentrations of a Group VIII metal selected from nickel and cobalt and relatively high concentrations of a Group VIB metal elected from molybdenum and tungsten. For arsenic removal from naphtha and light distillate cuts, nickel and molybdenum are preferred.
To demonstrate superior effectiveness in the removal of arsenic, the finished catalyst of the present invention should contain at least about 8 wt. % of the Group VIB metal. The amount of Group VIII metal deposited on the catalyst should be sufficient to result in an atomic ratio of the Group VIII metal to the Group VIB metal between 1.5/1 and 2.5/1, preferably 2/1.
Phosphorus aids hydrogenation. Its presence in the finished catalyst is also optional. If present, the amount of phosphorus should be between about 0.1 wt. % and about 3 wt. % of the finished catalyst.
Catalyst Preparation
The catalyst of the present invention may be prepared by first making a porous refractory support by any of the methods well known to those skilled in the art. In the preferred method, alumina powder is mixed with water and either acetic or nitric acid, and optionally with about 1 wt. % Group VIII metal as nickel or cobalt nitrate. The mixture is mulled and extruded into the desired shape. The support is finished by drying at approximately 100xc2x0 C. for several hours followed by calcining for 2 hours at a temperature of between 450xc2x0 C. and 700xc2x0 C.
The support is then impregnated with a sufficient amount of a solution of a Group VIII metal to result in the impregnated support containing at least 10 wt. % of the metal. The preferred metal is nickel and the preferred solutions are nickel nitrate in water and nickel carbonate in ammonium hydroxide. After impregnation, the support is dried for several hours at low temperature, i.e., approximately 100xc2x0 C., followed by calcination for at least two hours at a temperature of at least 427xc2x0 C., preferably at 510xc2x0 C.
The final step in preparing the catalyst is impregnation with a sufficient amount of solution containing a Group VIB metal compound and, optionally, an additional amount of the same Group VIII metal compound previously deposited so that the finished catalyst contains at least 8 wt. % of the Group VIB metal, and preferably approximately 13 wt. %. The impregnation solution may optionally be a phosphoric acid solution containing sufficient phosphorus to result in approximately 2 wt. % phosphorus on the finished catalyst. Following impregnation, the catalyst is dried for several hours at approximately 100xc2x0 C. and calcined at a temperature at least 30xc2x0 C. lower than the first calcination temperature for approximately two hours.
Process for Removing Arsenic from Hydrocarbon Fractions
Removal of arsenic from hydrocarbon fractions may be achieved by contacting such feed with the catalyst of the present invention in the presence of hydrogen at elevated temperature and pressure. The preferred operating conditions are between about 2,860 kPa and about 18,720 kPa, between about 260xc2x0 C. and about 490xc2x0 C., a hydrogen treat gas rate between about 180 m3/m3 of feed and 1,600 m3/m3 of feed and sufficient catalyst to result in a liquid hourly space velocity (LHSV) of between about 0.1 hrxe2x88x921 and about 40 hrxe2x88x921. It is preferred that the catalyst of the present invention be employed in the lead or first bed of a multi-bed system to permit removal of arsenic by a catalyst that is particularly well-adapted for such, thereby permitting the use of another catalyst less resistant to rapid deactivation from arsenic contamination in subsequent beds.